Modified porous materials and methods of creating interconnected porosity in materials

ABSTRACT

Techniques, mixtures and improved porous materials (interconnected porous constructs) that are capable of maintaining a sufficient porosity while conferring improved mechanical and physical strength to the final construct. A sacrificial construct (for example, a sacrificial material such as polymethyl methacrylate (PMMA)) is used to obtain an inverse porosity of the construct it was molded into. The process provides a less porous end material that may be used as an arthroplasty device or surgical implant (for example, an interference screw of suture anchor) among many other applications. The process employs a sacrificial material to reduce the porosity of the final construct to about 35%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 14/152,507, filedJan. 10, 2014, which claims the benefit of U.S. Provisional ApplicationNo. 61/770,703, filed Feb. 28, 2013, the disclosures of which areincorporated by reference in their entireties herein.

FIELD OF THE INVENTION

The present invention relates to the field of surgery and, moreparticularly, to modified porous materials and improved technologies forcreating and controlling porosity in materials.

BACKGROUND OF THE INVENTION

Porous materials are widely used in biomedical and industrialapplications. In the biomedical field, porous materials have been usedas scaffolds (templates) for tissue engineering/regeneration, wounddressings, drug release matrices, membranes for separations andfiltration, absorbents and hemostatic devices, among many others.

Despite their extensive use, the development of porous materials hasbeen affected due to the difficulty of making an article porous andkeeping it so while providing the material (article) with adequatestrength. For example, porous polymer materials have been used asscaffolds for cell incorporation, proliferation and tissue regenerationin aqueous environments (such as in a tissue culture medium, orimplanted inside a human or animal body). Yet, such porous polymers mustalso possess sufficient mechanical strength to withstand anatomicalpressures and deformations, and also be capable of maintaining theirstructure and function when undergoing various changes (for example,mechanical or environmental changes).

A need exists for techniques for creating and controlling porosity inmaterials to obtain a less porous material than the one obtained bytraditional methods. Also needed are methods of forming a porousmaterial with a modified, interconnected porosity that renders thematerial sufficiently strong and substantially stable in a predeterminedenvironment. Also needed is an implant that is at least partially porousto provide a scaffold for bone cells to grow into, but is also strongenough to withstand fabrication processes such as machining and/ormolding, among many others.

BRIEF SUMMARY OF THE INVENTION

The present invention provides techniques, mixtures and improved porousmaterials (interconnected porous constructs) that are capable ofmaintaining a sufficient porosity while conferring improved mechanicaland physical strength to the final construct.

The present invention uses a sacrificial construct (for example, asacrificial material such as polymethyl methacrylate (PMMA)) to obtainan inverse porosity of the construct it was molded into. The processprovides a less porous end material that may be used as an implant (forexample, an arthroplasty device, screw or anchor) among many otherapplications. The process employs a sacrificial material to reduce theporosity of the final construct to about 35%.

Other features and advantages of the present invention will becomeapparent from the following description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an image of merged PVA particles to form aninterconnected construct according to an exemplary embodiment of thepresent invention.

FIG. 2 illustrates an image of PMMA molded into the interconnectedporous PVA construct of FIG. 1.

FIG. 3 illustrates an image of porous PMMA sample after dissolving PVA.

FIG. 4 illustrates an image of a casting material molded into the porousPMMA sample of FIG. 3.

FIG. 5 illustrates an image of a casting material porous sample afterdissolving PMMA using alcohol.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides techniques and mixtures for creating andcontrolling porosity in materials to obtain a less porous material thanthe one obtained by traditional methods. The final porous material has amodified, interconnected porosity that renders the materialsubstantially stable in a predetermined environment.

The present invention uses a sacrificial construct (for example, asacrificial material such as polymethyl methacrylate (PMMA)) to obtainan inverse porosity of the construct it was molded into (to reduce theporosity from about 65% to about 35%). The process provides a lessporous end material that may be used as a surgical implant, anarthroplasty device, an interference screw, an anchor such as a sutureanchor, an implant (for example, a femoral implant such as a PLLAscrew), among many other applications.

The present invention provides a method of creating a porous material byinter alia the steps of: (a) creating a porous structure from a solublematerial; (b) filling the void area of the soluble porous structure witha non-soluble material; (c) curing the soluble and non-soluble material;(d) removing the soluble material to obtain a newly-formed porousmaterial; (e) filling the void area of the newly-formed soluble porousstructure with a non-soluble material; and (f) removing the solublematerial to obtain a porous material.

The present invention also provides a method of creating a porousmaterial by inter alia the steps of: (a) creating an interconnectedporous structure from a water soluble material (the water solublematerial may be a hydrogel such as polyvinyl alcohol); (b) filling thevoid area of the water soluble interconnected porous structure with anon-soluble material (the non-soluble material may be an acrylic such aspolymethyl methacrylate (PMMA)); (c) curing the soluble and non-solublematerial; (d) removing the water soluble material to obtain anewly-formed interconnected porous material; (e) filling the void areaof the newly-formed soluble interconnected porous structure with anon-soluble material (the non-soluble material may be a bioresorbablematerial); and (f) removing the soluble material to obtain aninterconnected porous material (the soluble material can be removed byan organic solvent, the organic solvent can belong to an alcohol groupsuch as isopropyl or ethanol; the organic solvent can belong to an ethergroup; the organic solvent can belong to an vinyl group).

An exemplary method of forming an improved interconnected porousconstruct according to an embodiment of the present invention isdetailed below with reference to FIGS. 1-5. The process detailed belowwith reference to FIGS. 1-5 may be conducted with steps similar to themethod of producing a porous construct as detailed in U.S. Pat. No.7,943,677, issued May 17, 2011, the disclosure of which is incorporatedin its entirety by reference herein. As detailed in U.S. Pat. No.7,943,677, a solvent merging/particulate-leaching method is used toproduce interconnected volumetric porosity.

An illustrative embodiment of the invention includes a method ofproducing a porous construct by creating negative porosity twice in thesame process. In a first step, a plurality of soluble particles aremerged in the presence of a solvent, e.g., an inorganic acid such aphosphoric acid or a compound. In one embodiment, the compound comprisesabout 5% phosphoric acid and 95% distilled water. In a preferredembodiment, the plurality of soluble particles are water soluble, suchas PVA.

The soluble particles are then dried for about 24 hours and a firstcasting material (such as PMMA) is added to fill the void area of thewater soluble PVA material. Curing of the first casting material and theplurality of soluble polymer particles for about 48 hours results in apolymerized construct.

Once cured, the construct is exposed so that the water soluble particlesare dissolved away. In one embodiment, the soluble particles aredissolved in warm water between about 85° Celsius and the T_(g)° of thecross-linking polymer. Preferably the soluble particles are dissolved ina solvent, such as water, at a temperature below the T_(g)° of the firstcasting material. Removal of the water soluble particles produces anewly-formed interconnect porous structure of the first non-solublecasting material (PMMA), i.e., by dissolving the merged solubleparticles, interconnected pores are produced throughout at least avolumetric portion of the first non-soluble casting material forming aporous construct.

The above steps set forth in ¶[0019]-¶[0021] are repeated for fillingthe void area of the interconnect porous structure of the firstnon-soluble casting material (PMMA) with a second non-soluble castingmaterial, which is then subjected to removal of the first non-solublecasting material (PMMA) to obtain a final interconnected porous materialformed of the second non-soluble casting material with a porosity ofabout 35%.

The second casting material may be any material that is cast and belowthe melting temperature of the other material. For example, the castingmaterial may be any degradable or biodegradable polymer (or combinationof such polymers) such as PLLA (poly-L-lactide acid or poly-L-lacticacid), PLA (poly lactic acid), PLGA (co-poly lactic acid/glycolic acid),simple linear polymers such as PGL (polyglycerol), PGA (polyglycolicacid) or caprolactone-based polymers such as PCL, among many othermonomers, polymers, block copolymers, linear polyesters, etc. In anexemplary-only embodiment, and as detailed below, PLLA is employed as anexemplary material to explain the process of the present invention.

PVA is preferred to create porosity in the materials for its fastdissolving rate. The process employs a mild phosphoric acid.

An exemplary method of the present invention is set forth below withreference to FIGS. 1-5 and by the following steps:

-   -   1) Creating an interconnected porous structure 10 (FIG. 1) by        merging a plurality of water soluble particles (PVA) using a        mild phosphoric acid;    -   2) Drying the plurality of water soluble PVA particles;    -   3) Filling the void area of the water soluble interconnected        porous structure with PMMA to form construct 20 (FIG. 2);    -   4) Removing the water soluble material to obtain a newly-formed        interconnected porous PMMA material 30 (FIG. 3);    -   5) Filling the void area of the newly-formed soluble        interconnected porous structure with casting material (for        example, PLLA) to form construct 40 (FIG. 4); and    -   6) Removing the soluble material using isopropyl alcohol to        obtain an interconnected porous material 100 (FIG. 5).

According to another exemplary-only embodiment, a method of creatinginterconnected porosity in materials comprises the steps of: (i) merginga plurality of soluble particles; (ii) applying a non-soluble castingmaterial to the merged soluble particles; (iii) curing the non-solublecasting material and merged soluble particles; (iv) dissolving themerged soluble particles; (v) applying another non-soluble castingmaterial to the porous construct; (vi) curing the non-soluble castingmaterial and porous construct; and (vii) dissolving the porousconstruct.

According to yet another exemplary-only embodiment, a method of formingan interconnected porous material comprises the steps of: (i) providinga porous structure of a first material having a first solubility; (ii)applying a second material to the porous structure, the second materialhaving a second solubility which is different from the first solubility;(iii) curing the first and second materials; (iv) removing the firstmaterial to obtain a newly-formed porous structure, the first materialbeing removed with a first biocompatible solvent that affects the firstmaterial but not the second material; (v) applying a third material tothe newly-formed porous structure, the third material having asolubility different from that of the first and second materials; (vi)curing the third material and the newly-formed porous structure; and(vii) removing the second material to obtain the final interconnectedporous construct, the second material being removed with a secondbiocompatible solvent that affects the second material but not the thirdmaterial.

The methods of the present invention have particular applicability tothe formation and fabrication of a variety of constructs and implants,for example, arthroplasty devices and implants, interference screws(such as exemplary PLLA screws), suture anchors, i.e., any variety ofconstructs that are at least partially porous (about 35% porosity) toprovide a scaffold for bone cells to grow into, but are also strongenough to withstand fabrication processes such as machining and/ormolding, among many others (to form the final implant/construct).

The present invention provides methods and porous structures that areformed by creating negative porosity twice during the process and bycarefully selecting the materials and solvents corresponding to eachstep. For example, and as detailed above, the second material must beselected so that it can allow for it to be selectively removed withoutaffecting the casting material, i.e., the third material; the secondmaterial must be also biocompatible. Similarly, the solvent of the firstmaterial must be selected so that it cannot affect the solubility andremoval rate of the other materials and must be also biocompatible.Thus, to allow the solvent merging/particulate-leaching process to beconducted twice, all materials must be carefully selected based on thesolubility/removal rate relative to each other.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, embodiments andsubstitution of equivalents all fall within the scope of the invention.

What is claimed is:
 1. A construct comprising an interconnected porousstructure having about 35% porosity and comprising polymethylmethacrylate and a polymer selected from the group consisting ofpoly-L-lactic acid (PLLA), polylactic acid (PLA), co-polylacticacid/glycolic acid (PLGA), polyglycerol (PGL), polyglycolic acid (PGA),polycaprolactone (PCL), and combinations thereof, wherein the constructis a surgical implant.
 2. A construct comprising an interconnectedporous structure with inverse porosity comprising a sacrificial materialand a degradable polymer.
 3. The construct of claim 2, wherein thesacrificial material is polymethyl methacrylate.
 4. The construct ofclaim 2, wherein the construct is an implant.
 5. The construct of claim2, wherein the construct is a screw.
 6. The construct of claim 5,wherein the screw is an interference screw.
 7. The construct of claim 2,wherein the construct is a suture anchor.
 8. The construct of claim 2,where in the porosity is about 35%.
 9. The construct of claim 2, whereinthe degradable polymer is biodegradable.
 10. The construct of claim 2,wherein the degradable polymer is selected from the group consisting ofpoly-L-lactic acid (PLLA), polylactic acid (PLA), co-polylacticacid/glycolic acid (PLGA), polyglycerol (PGL), polyglycolic acid (PGA),polycaprolactone (PCL), and combinations thereof.